1. Field of the Invention
The present invention generally relates to a redundant light source switching method and a wavelength division multiplexing transmission apparatus using such a method. More particularly, the present invention is concerned with a redundant switching method for a working light source in a wavelength division multiplexing transmission system and a wavelength division multiplexing transmission apparatus using the same.
In a high-capacity very-high-speed wavelength division multiplexing transmission system such as a D-WDM (Dense-Wavelength Division Multiplexing), there are provided a double or triple redundant system because the system is greatly affected by occurrence of a failure. It is required, in the event of a failure, that communication services can be restored as shortly as possible.
2. Description of the Related Art
Generally, a semiconductor laser diode, which is a lifetime-limited component, is used as a light source provided on the transmission-line side of the WDM transmission apparatus. Conventionally, only the lifetime of the laser diode by checking a drive current applied thereto is supervised within the WDM transmission apparatus. That is, the conventional WDM transmission apparatus is not equipped with a redundant system capable of immediately coping with a failure of the laser diode. Conventionally, there are the three redundancies in the laser diode.
The first redundancy in the laser diode prepares protection or spare light source packages in a given office. If a working light source becomes faulty, a protection light source package having the same wavelength as the working light source is substituted therefor.
The second redundancy in the laser diode prepares working and protection packages for the respective wavelengths in the WDM transmission apparatus. If a fault occurs in the working package, it is manually replaced by the protection package.
The third redundancy in the laser diode prepares a wavelength-variable protection light source. If a fault occurs, the protection light source is substituted for the faulty light source and is tuned to the wavelength used in the working light source.
However, the first through third redundancies have the following disadvantages.
The first redundancy cannot restore communications soon. Usually, due to an economic issue, protection packages are not provided in all the offices but specific offices located in the respective maintenance areas. Thus, in an event of an office or station failure, it is required to order a protection package from the corresponding specific office located far away therefrom. Thus, it takes a long time to restore communications.
The second redundancy is not economical because each of pairs of packages (working and protection packages) must be provided to the respective wavelength. That is, the number of packages must be doubled in order to implement the second redundancy. Thus, the second redundancy is expensive. In addition, the WDM transmission apparatus requires an increased mounting space.
The third redundancy has a disadvantage in that it is difficult to perform the wavelength control (setting) in a wide wavelength band. Particularly, it is difficult to obtain, by using the wavelength-variable protection light source, almost the same characteristics as the working light source of the given wavelength at the same accuracy as that of the working light source.